An increase in density and in the degree of integration of electronic elements such as transistors, integrated circuits, memory elements, or the like recently observed in the design of printed circuit boards and hybrid ICs are accompanied by the use of thermally conductive silicone rubber compositions that are characterized by more efficient heat radiation properties.
Such thermally conductive silicone rubber compositions may be exemplified by the following: a thermally conductive silicone rubber composition comprising an organopolysiloxane with vinyl groups, an organohydrogenpolysiloxane, a thermally conductive filler, aminosilane, an adhesion-imparting agent selected from epoxy silane or alkyltitanate, and a platinum-type catalyst (see Japanese Unexamined Patent Application Publication No. (hereinafter referred to as “Kokai”) S61-157569); a thermally conductive silicone rubber composition comprising a diorganopolysiloxane that contains in one molecule an average of two alkenyl groups, an organopolysiloxane that has in one molecule an average of three or more silicon-bonded hydrogen atoms, a thermally conductive filler composed of zinc oxide and magnesium oxide, a surface-treating agent for a filler, and a platinum-type catalyst (see Kokai S62-184058); a thermally conductive silicone rubber composition comprising an organopolysiloxane that contains in one molecule at least 0.1 mole % of alkenyl groups, an organohydrogenpolysiloxane that contains in one molecule at least two silicon-bonded hydrogen atoms, a spherical alumina powder with an average particle size in the range of 10 to 50 μm and a spherical or aspherical alumina powder with an average particle size below 10 μm, and platinum or a platinum-type compound (see Kokai S63-251466); a thermally conductive silicone rubber composition comprising an organopolysiloxane with alkenyl groups, an organohydrogenpolysiloxane, an amorphous alumina powder with an average particle size in the range of 0.1 to 5 μm and a spherical alumina powder with an average particle size in the range of 5 to 50 μm, and a platinum-type catalyst (see Kokai H2-41362); and a thermally conductive silicone rubber composition comprising an organopolysiloxane having in one molecule at least two alkenyl groups, an organohydrogenpolysiloxane having in one molecule at least three silicon-bonded hydrogen atoms, a thermally conductive filler with an average particle size in the range of 5 to 20 μm, an adhesion-assisting agent, and a platinum or platinum-type catalyst (see Kokai H2-97559).
However, in order to improve thermal conductivity in a cured body obtained from such thermally conductive silicone rubber compositions, the latter must incorporate a large amount of a thermally conductive filler, but an increase in the amount of such a filler impairs handleability and moldability of the composition, and also worsens physical properties in products molded from such compositions. Another drawback is low adhesion of the composition to various substrates during curing.
In view of the above, it was proposed to improve handleability and moldability of a thermally conductive silicone rubber composition (1) by preparing this composition from an organopolysiloxane that contains in one molecule at least two alkenyl groups, an organohydrogenpolysiloxane that contains in one molecule at least two silicon-bonded hydrogen atoms, an organopolysiloxane that contains in one molecule at least one silicon-bonded alkoxy group or silicon-bonded hydroxyl group, a fine spherical or aspherical alumina powder with an average particle size below 10 μm and a fine spherical or aspherical alumina powder with an average particle size in the range of 10 to 50 μm, and a hydrosilylation catalyst (see Kokai H8-325457); (2) by preparing the composition from an organopolysiloxane, a methylpolysiloxane that contains a hydrolyzable group, a thermally conductive filler, and a curing agent (see Kokai 2000-256558); or (3) by preparing the composition from an organopolysiloxane, a curing agent, and a thermally conductive filler surface-treated with an oligosiloxane having silicon-bonded alkoxy groups (see Kokai 2001-139815).
However, when the thermally conductive compositions that are mentioned above incorporate a large amount of a thermally conductive filler such as alumina for improving thermal conductivity in a silicone rubber obtained by curing the composition, they become extremely viscous, difficult to handle and to mold, and therefore cannot be applied satisfactorily for use, e.g., in stencil printing. Other problems associated with the aforementioned compositions are decrease in adhesive properties, elongation characteristics, and tensile strength of the silicone rubber obtained by curing the composition. On the other hand, in order to improve the adhesive properties, elongation characteristics, and tensile strength of silicone rubber, the latter should be combined with a reinforcing filler, but an addition of the reinforcing filler adversely affects thermal conductivity of the silicone rubber.
It is an object of the present invention to provide a thermally conductive silicone rubber composition that demonstrates good handleability and possesses appropriate thixotropicity even in the presence of a large amount of a thermally conductive filler and that, when cured, produces a thermally conductive silicone rubber characterized by high tensile strength and improved adhesion and elongation characteristics even without the use of a reinforcing filler.